Fiberboards, including corrugated and noncorrugated paperboards, are useful for an extremely wide variety of applications, but particularly for making containers such as packaging and shipping containers. Modern techniques for making such containers involve not only manufacturing the requisite fiberboard material but also cutting and shaping of one or more sheets of the fiberboard into "box blanks" that are folded into the corresponding container shape. Box blanks are typically designed with multiple scored lines and the like so that the blank can be readily formed into a container by merely folding the box blank in an ordered manner along the scored lines. Regardless of the container design, the forming of a substantially planar box blank into a corresponding three-dimensional container involves subjecting the fiberboard to a plurality of folds.
One drawback to many fiberboards, including paperboard, is their poor rigidity when wet. To overcome this shortcoming, manufacturers have tried various ways of reinforcing fiberboard and rendering the fiberboard nonabsorptive for liquids. Examples of such reinforcement include impregnating or coating the fiberboard with paraffin or other polymeric material.
Paraffin coating substantially decreases the tendency of the fiberboard to absorb water, making paraffin-reinforced corrugated paperboard popular for use in packaging vegetables and meats. Unfortunately, paraffin has the disadvantage of being readily softened by moderately elevated temperatures. Also, while paraffin coating can sometimes enhance the compressive strength of the fiberboard and resistance to puncturing, the enhancement may not be sufficient for many uses. In view of the shortcomings of reinforcing fiberboard using paraffin, other polymeric resins, particularly various thermoset materials, have been considered for this purpose. Many cured thermosets have the advantage of being very rigid. As a result, fiberboards reinforced with cured thermosets tend to have high resistance to compression. Unfortunately, many currently favored thermosets are extremely brittle after being fully cured and fracture when subsequently creased or folded. Such fracturing of the thermoset reinforcing agent can readily extend to the fiberboard itself, thereby seriously reducing the integrity of the container made therefrom along edges and at corners.
Phenolics have received the greatest attention, particularly as a reinforcing agent for corrugated paperboard. Representative U.S. Pat. Nos. disclosing use of phenolics include U.S. Pat. Nos. 3,886,019, 4,096,935, 4,051,277, and 4,096,305 to Wilkenson et al. These patents disclose the application of thin films of phenolic resin to surfaces of linerboards and corrugated medium that will be adhered together to form the corrugated paperboard. After adhering together the linerboards and corrugated medium, the corrugated paperboard can be cut, scored, and slotted to make box blanks. Because of the brittleness of the fully cured treated board, full curing of the resin is delayed until after the box blanks have been folded to make cartons.
Various thermoset blends of phenolics with other resins have also been tried in an attempt to reduce the brittleness of phenolic alone. Representative U.S. patents include Reisman et al U.S. Pat. Nos. 3,687,767 (phenolaldehyde), LeBlanc et al. U.S. Pat. No. 3,607,598 (phenol-aldehyde plus polyvinylalcohol), Reisman U.S. Pat. No. 3,616,163 (phenolaldehyde resole), Elmer U.S. Pat. No. 3,619,341 (phenol-aldehyde resole), Burke U.S. Pat. No. 3,619,342 (phenol-aldehyde resole), Reisman et al. U.S. Pat. No. 3,697,365 (resole phenolic plus an organosilyl compound), LeBlanc U.S. Pat. No. 3,682,762 (resole phenolic plus polyaminoalkyl substituted organosiloxane), LeBlanc U.S. Pat. No. 3,617,427 (aminoplast-modified phenol-aldehyde resole), Carlson U.S. Pat. No. 3,617,428 (aminoplast with phenol-aldehyde resole), and LeBlanc U.S. Pat. No. 3,617,429 (aminoplast plus phenol-aldehyde and polyvinylalcohol).
Despite these developments, even phenolic blends tend to be unacceptably brittle, which imposes certain limitations on manufacturing processes. For example, in all the phenolic-blend patents recited above, curing (thermosetting) of the resin is performed only after corrugating the medium fiberboard or even later such as after the corrugated paperboard is scored along fold lines. This means, for example, that resin-coated paperboard destined to become the corrugated medium cannot be cured before it is passed through a corrugating machine. As a result, conventional thermoset-impregnated medium paperboard cannot be made up and cured in one location and supplied to another location for corrugating and incorporation into corrugated paperboard using conventional machinery. Also, interposition of resin-applying and resin-curing machinery into existing production lines for manufacturing corrugated paperboard is expensive. These and other problems with existing methods can result in prohibitively high production and shipping costs.